I may be vociferosuly agreeing here, but isn't it comparing body parts that both the defender and attacker can "use"? eg an armpit v a a buttock...both can touch the ball legally, so if your armpit is 1mm beyond the defender's butt cheek, you're technically offside.
What's happening is that VAR is exposing some of the weird laws of the game, I think. (it has other problems too)